Within a Voice over Internet Protocol (VoIP) network, such as an Internet Protocol (IP) Multimedia Subsystem (IMS) network, subscribers that register in that network will be allocated to a particular call session establishing server (such as a Serving Call Session Control Function (S-CSCF) server) of that network. The IMS network may contain one or multiple S-CSCF servers. Selection of a S-CSCF server for the subscriber is based on S-CSCF server capability. When the subscriber performs an initial registration, the Home Subscriber Server (HSS) informs the Interrogating Call session Control Function (I-CSCF) server about the S-CSCF capability required for this subscriber. The I-CSCF server then selects a S-CSCF server from a locally (within the I-CSCF serer) configured table of S-CSCF servers with supported capabilities.
The selection of the S-CSCF server for the subscriber is static. Once the subscriber is registered, the subscriber remains registered in that S-CSCF server, until the subscriber would de-register (explicitly by the subscriber or implicit through registration expiry). Also, when a failure occurs with the S-CSCF server, the subscriber may end up being registered in a different S-CSCF server.
Assuming that S-CSCF server failure should occur as infrequent as possible and further assuming that subscribers may remain registered for a long duration, a subscriber may end up being registered in a particular S-CSCF server for a long duration.
When the registered subscriber establishes a communication session such as a Voice call, either originating call or terminating call, then there will be a Session Initiation Protocol (SIP) session established with a further IMS subscriber. That further IMS subscriber may be registered in the same S-CSCF server or in a different S-CSCF server. Signaling related to this SIP session will traverse the S-CSCF server of the calling party as well as the S-CSCF server of the called party.
Signaling related to the initial Invite transaction of this SIP session will also traverse an I-CSCF server. The S-CSCF server and the I-CSCF server may be located in the same host (network node). A S-CSCF server may, when handling an initial Invite transaction, select the co-located I-CSCF server, provided that the destination party of the SIP session belongs to the same network. When the calling party and the called party are registered in the same S-CSCF server, then this will require less signaling for the initial Invite transaction related signaling compared to the case where calling party and called party are not registered in the same S-CSCF server, since signaling will be purely within the node.
SIP signaling within the SIP session after the 200 Ok (in-session signaling), i.e. when the SIP session has become an established SIP session, will not traverse the I-CSCF server, but will run directly from the S-CSCF server associated with the calling party to the S-CSCF server associated with the called party. When the calling party and the called party are registered in the same S-CSCF server, then this results in less signaling for the in-session transaction compared to the case where calling party and called party are not registered in the same S-CSCF server, since signaling will be purely within the node.